Combined Superbase Ionic Liquid Approach to Separate CO2 from Flue Gas

Superbase ionic liquids (ILs) with a trihexyltetradecylphosphonium cation and a benzimidazolide ([P66614]­[Benzim]) or tetrazolide ([P66614]­[Tetz]) anion were investigated in a dual-IL system allowing the selective capture and separation of CO2 and SO2, respectively, under realistic gas concentrations. The results show that [P66614]­[Tetz] is capable of efficiently capturing SO2 in preference to CO2 and thus, in a stepwise separation process, protects [P66614]­[Benzim] from the negative effects of the highly acidic contaminant. This results in [P66614]­[Benzim] maintaining >53% of its original CO2 uptake capacity after 30 absorption/desorption cycles in comparison to the 89% decrease observed after 11 cycles when [P66614]­[Tetz] was not present. Characterization of the ILs post exposure revealed that small amounts of SO2 were irreversibly absorbed to the [Benzim]− anion responsible for the decrease in CO2 capacity. While optimization of this dual-IL system is required, this feasibility study demonstrates that [P66614]­[Tetz] is a suitable sorbent for reversibly capturing SO2 and significantly extending the lifetime of [P66614]­[Benzim] for CO2 uptake.